1. Field of the Invention
The present invention relates to a piezoelectric ink jet printer, and more particularly to a printing head for such a printer wherein a laminated piezoelectric device is used as a piezoelectric actuator.
2. Description of the Related Art
A piezoelectric ink jet printing head has recently been proposed in the art. The head is primarily comprised of an ejection device and a piezoelectric actuator. The ejection device has an ink chamber whose volume is changed depending on the displacement of the piezoelectric actuator. In a printer known as a drop-on-demand type, when the volume of the ink chamber is reduced, ink contained in the ink chamber is ejected through a valve defining the ink chamber whereas when the volume of the ink chamber is increased, ink is supplemented into the ink chamber through another valve which also defines the ink chamber. A multiplicity of the ejection devices are closely juxtaposed so that a desired character or image is formed by ejecting ink droplets from selected ejection devices.
A conventional drop-on-demand printer head uses a single piezoelectric actuator for a single ejection device. Although it is desirable to include a number of ejection devices in the printer head so as to be capable of printing over an extensive area with high resolution, there have been difficulties in so doing in the conventional head structure. Partly because the structure of the head becomes complicated, and partly because the manufacture of the head becomes intricate, thus the manufacture of the head becomes costly due to a large number of manufacturing steps involved. Further, the dimension of the piezoelectric actuator cannot be made so small due to machining reasons.
To solve such problems, applicants have proposed an improved printer head using a piezoelectric actuator of a laminated structure as disclosed in U.S. application Ser. No. 07/673,148 filed Mar. 21, 1991, abandoned. In the laminated structure, piezoelectric ceramic layers and internal electrode layers are alternately arranged one on the other. At least one of the internal positive electrode layers and the internal negative electrode layers, both constituting the internal electrode layers, is divided into a plurality of segments so as to be provided in one-to-one correspondence to the respective ones of the ejection device, whereby a high resolution, low-voltage driven printer head can be provided which is simple in structure and inexpensive in cost.
The printer head using a laminated piezoelectric actuator (LPA) 38 is shown in FIG. 1. As shown, the LPA 38 is formed with three piezoelectric active regions 46a, 46b, 46c and four piezoelectric inactive regions 48. An ink channel body 34 is secured to the LPA 38 at the piezoelectric inactive regions 48. An orifice plate 36 formed with orifices 37a, 37b, 37c is secured to the opposite side of the ink channel body 34. When a driving voltage is applied between an external negative electrode 52 and an external positive electrode 54a, the piezoelectric active region 46a is deformed in the direction of its thickness as shown. The volume of the associated ink channel 32 is thereby reduced and thus an ink droplet is ejected from the orifice 37a. The LPA 38 is provided for a plurality of ejection devices 70a, 70b, 70c. Since the LPA 38 is made up of a reduced number of components, its structure is relatively simple. Further, due to the use of LPA 38 having segmental internal electrodes, the device can be driven at a low voltage, yet capable of printing with high resolution.
In the laminated piezoelectric structure as described, there is a problem that an amount of displacement of the piezoelectric active region is adversely reduced if the active region is widened intending to drive it with a lower driving voltage. In order to have the same amount of displacement while widening the active region, the driving voltage must be increased. Further, since the piezoelectric active region displaces the same amount toward not only the ink channel but also its opposite direction, the ejection of the ink droplet is performed with a half of full energy.
In the printer head of the type described above, the LPA 38 is extended pursuant to a longitudinal effect. The displacement.times.of the LPA 38 is represented by the following equation: EQU x=(d.sub.33 .times.V).times.n
where d.sub.33 is a piezoelectric constant pursuant to the longitudinal effect, V is a driving voltage, and n is the number of laminated layers. As can be appreciated from the above equation, if the number n of the laminated layers is increased, the required driving voltage to attain a desired amount of displacement can be reduced and hence a low-voltage driven piezoelectric ink jet printer head can be provided. However, in order to make the printer head compact, the thickness of the LPA 38 needs to be reduced. In order to increase the number of laminated layers and to reduce the thickness of the LPA 38, it is absolutely necessary to reduce the thickness of each layer of the LPA 38. An electrostatic capacity C of the LPA 38 is represented by: EQU C=.epsilon..multidot.S/t
where .epsilon. is a dielectric constant, S is an entire area of the internal electrode, and t is the thickness of each piezoelectric layer. As is apparent from the above relation, the electrostatic capacity of the LPA 38 increases as the number of laminated layers increases and as the thickness of the piezoelectric layer becomes thinner. While it is theoretically possible to drive the piezoelectric ink jet printer head at a low voltage, say several volts, if the thickness of one layer is made thinner and the number of laminated layers is increased, it is required that extremely high-level current be instantaneously flowed to instantaneously deform the LPA 38 due to an extremely large electrostatic capacitance. In the piezoelectric ink jet printer head, the ink droplet is generally ejected at a time when the increased volume of the ink chamber is restored. Although it is possible to control the instantaneously flowed current by delaying the rising time of the voltage across the piezoelectric layer or by the use of an LC resonance circuit, it is necessary that the change of the pressure in the ink chamber instantaneously occur. The deformation of the LPA or the discharge of the LPA 38 must occur within several microseconds. As such, an expensive discharge circuit is required due to the fact that the maximum instantaneous current at the time of discharge is extremely large.
An electrostatic capacitance of one piezoelectric driving portion is about 35 nF. The volume change of the ink chamber necessary for ejecting ink droplets is about 3.37.times.10.sup..about.4 mm.sup.3, and a driving voltage to achieve the necessary volume change of the ink chamber is about 20 volts. According to a conventional driving sequence, a pulse voltage of minus 20 volts is applied across the piezoelectric layer so that the voltage thereacross reaches minus 20 volts within a duration of 3 .mu.sec to thus deform the piezoelectric element, and the same voltage is further applied thereacross for a duration of another 10 .mu.sec. Thereafter, the piezoelectric layer is subjected to discharge within a duration of 2 .mu.sec so as to be restored to the original state. The head is driven at a frequency of 10 kHz at maximum while taking print quality into consideration. The printing period of the head is thus 100 .mu.sec at the shortest. According to the above-described driving sequence, it is required that an extremely large instantaneous current such that its maximum level is about minus 470 mA flowed into the piezoelectric layer in order to increase the voltage across the 35 nF piezoelectric element up to minus 20 V within a duration of 3 .sigma.sec. If 64 driving portions in the piezoelectric actuator are simultaneously driven, a current whose maximum level is about minus 30 amperes needs to be instantaneously supplied. Hence, there is a problem that a power source having a capability of supplying 600 VA power needs to be provided for driving the head.
In actuality, however, It is extremely rare that 64 driving portions are simultaneously driven to eject ink droplets from the 64 orifices at a time, thus the provision of such a large power source is redundant. Nevertheless, such a large power source is provided. Consequently, due to the presence of the large power source, the size of the ink jet printer head cannot be made compact and the cost of the printer head is increased.